Hitchhiker's Guide to the Internet is a German introduction to the technical aspects of the Internet. This book explains both the low-level protocols IP, ICMP, TCP, and UDP and the high-level protocols SMTP, POP3, IMAP4, NNTP, HTTP, Gopher, FTP, IRC, DICT, Finger, Daytime, DNS, and Whois.
Mailvisa is a spam filter along the lines of Paul Graham's "A Plan for Spam". It classifies messages by comparing the words in them to known spam words. CPU usage, filtering speed, and filtering accuracy are comparable to other Bayesian filters. Mailvisa was originally intended as a platform for experimentation with different filtering parameters, but it has evolved into a useful spam classifier. In the author's own use, the amount of spam caught has surpassed 95% with no false positives so far.
rit can be used to manipulate mailboxes over IMAP, allowing users to copy and delete mailboxes and their contents. When copying a mailbox, its subscription status and all of the IMAP flags of the messages it contains are preserved on the destination server. Dynamic mailbox name translation can be performed if the mailboxes need to be renamed as they are copied.
Csv2vcard is a program for converting CSV (comma separated values, as exported by software such as MS Outlook) files containing contact information to the industry standard vcard format, which can be imported by most mailers and PIM software (optimized for Evolution). It's based on a Perl script by Michael MacDonald of Ximian. Csv2vcard can be used to convert any CSV format, not just the English Outlook output.